1. Field of the Invention
The present invention relates to a method for assigning a pilot PN(PseudoNoise) offset for a digital mobile telecommunications system, and in particular, to a method for assigning a pilot PN offset which can increase pilot PN offset re-use efficiency.
2. Description of the Related Art
In general, since frequency resources are limited in mobile telecommunications systems, these frequency resources should be re-used. In current analog mobile telecommunication systems, different frequencies are allocated to a mobile unit and a cell site. The frequencies used for the mobile unit and the cell site are divided into two groups: block A and block B.
FIG. 1a illustrates transmit frequencies and channel numbers for mobile units, and FIG. 1b illustrates transmit frequencies and channel numbers for cell sites. In such an analog mobile telecommunications system, 42 set-up channels and 624 voice channels are assigned as follows.
TABLE 1 channel number usage remarks 1-312 voice channel block A 313-333 set-up channel block A 334-354 set-up channel block B 355-666 voice channel block B
Due to the finite frequency spectrum, the frequency re-use concept is applied to mobile telecommunications systems by allocating different frequencies to adjacent cell sites, and the same frequency to cells that are remote from each other. As shown in FIG. 2, a cell site CS1 and its neighbor CS2 use different frequencies, whereas the cell site CS1 and its remote cell site CS3 use the same frequency. Here, the cell sites CS1 and CS3 should be too far away from each other to cause co-channel interference.
A frequency re-use distance D, for example, between the cell sites CS1 and CS3, which allows the same frequency channel to be re-used, can be determined from EQU D=3K+L .times.R (1)
where K is frequency re-use efficiency and R is the radius of a cell site. EQU K=i.sup.2 +i.j+j.sup.2 (2)
where i is the distance between adjacent cells (e.g., CS1 and CS2) and j is the distance between cells using the same channel frequency (e.g., CS2 and CS3)
The frequency re-use distance D depends on the number of co-channel cells in the vicinity of a cell, the type of geographic terrain contour, the antenna height, the transmitted power at each cell site, and a desired carrier-to-interference (C/I) ratio. If the frequency re-use distance D decreases, the frequency re-use efficiency K increases and the co-channel interference simultaneously increases. Hence, the challenge is to obtain the smallest number K which can still meet intended service quality. If D=3.46R, K=4, if D=4.6R, K=7, if D=6R, K=12, and if D=7.55R, K=19. The (D=4.6R, K=7) frequency re-use pattern is generally used.
The above frequency allocating method for analog mobile telecommunications systems is not feasible for digital mobile telecommunications systems. In general, a CDMA (Code Division Multiple Access) digital mobile telecommunications system uses pilot PN codes. Since there are a limited number of pilot PN codes (i.e., 512 (0-511)), all base stations cannot have different pilot PN codes. In pursuit of pilot PN code re-use under these circumstances, if a pilot increment (hereinafter, referred to as PT-inc) decreases, pilot code re-use efficiency increases, while the PN code and PN offset differences between adjacent base stations are smaller, resulting in interference. On the other hand, if the PT-inc increases, the PN offset difference between adjacent base stations decreases, thereby reducing interference, while the pilot code re-use efficiency drops.
The conventional frequency allocating method for an analog mobile telecommunications system cannot be applied to the PN code assignment for a digital mobile telecommunications system. This is because the conventional method considers only the re-use distance. In the digital mobile telecommunications system, delay-caused offset difference, as well as re-use distance should be considered.
A specific re-use pattern is not set for such a pilot code assigning method in the digital mobile telecommunications system. In general, a re-use PT-inc is set to 10 or 12, and a re-use distance is set to 6R (here, R is the radius of a base station), in order to prevent interference.
However, with a PT-inc of 10 in a three-sector digital mobile telecommunications system, an identical pilot PN code is used at every 17th base station, resulting in the decrease of the re-use efficiency and possible interference. With a PT-inc of 12, the re-use distance is 6.25R at maximum. In this case, as far as all cells have the same transmitted output and the same radius, in a flat area in terms of wave environment, no interference exists between cells sharing an identical pilot code. However, in implementing an actual wireless optimization, each base station has a different transmitted output, the cell radius is not uniform, and a signal delay due to multipath by 6R or larger in a line-of-sight condition may be introduced. Thus, handoff and call failures may result.